The present invention relates to a MacPherson-type suspension shown in FIG. 1 in which a strut 1 formed essentially of a shock absorber participates in the steering of the wheel.
The shock absorber comprises a body 2 and a rod 3. The upper end 30 of the rod rests on the body of the vehicle on an articulation point 31 formed, in general, by an elastic articulation. A coil spring 4 is mounted between an upper retainer 40 and a lower retainer 41. The lower retainer 41 rests on the body 2 of the shock absorber, and the upper retainer 40 rests on the body of the vehicle, in general also via a filtering effected by said elastic articulation, which filtering can, in certain cases, be the same for the spring and for the shock absorber.
The body 2 is firmly attached to a hub holder 5. There is therefore no degree of freedom between the body 2 and the hub holder 5. A lower arm 6 is articulated on one side on the body of the vehicle and on the other side on the hub holder 5 by the articulation 65. The hub holder 5 supports a hub 50 on which there is mounted a wheel 51 equipped with its tire 52.
In the zone of attachment to the body, the strut 1 exerts forces on the body, coming both from the spring and from the shock absorber. Although the exact point of application of these forces is difficult to locate and depends in particular on the specific construction of the strut 1, it is known that the mastering of the shearing force which the rod 3 of the shock absorber undergoes is determinative for the proper operation of suspensions of this type.
In order to simulate a MacPherson suspension, one can consider the reaction R of the body of the vehicle on the strut. Its direction passes through the point C, the latter being defined by the intersection of the force B exerted by the lower arm 6 on the articulation 65 and the resultant of the force S which the ground exerts on the tire 52. In first approximation, the vehicle being stationary, said resultant force S is directed vertically and passes through the center of the contact area of the tire (the value of this force upon travel in a straight line with stabilized speed depends on the characteristics of the tire and the adjustments of the geometry of the front axle). The intersection of B and S gives the point C. The reaction R is divided between the thrust P of the spring and a shearing force T applied to the rod 3 of the shock absorber in a direction perpendicular to said rod 3, so that the relationship R=T+P (vector sum) applies.
In order to obtain good operation of the strut (no or little dry friction, minimum wear, etc.), it is known that it is advisable to control the shearing force T applied to the rod 3 very closely. For this purpose, it has already been proposed to incline the axis of the spring 4 with respect to the axis of the rod 3, as shown in the drawing. Thus, practically all the MacPherson suspensions used at the present time comprise a spring, the axis of which is not identical with the axis of the rod of the shock absorber. The inclination of this spring 4 has been calculated in order to minimize the shearing force on the rod 3.
Furthermore, research has shown that the resultant of the forces exerted by the spring can exert a torque Cp around the pivot axis, defined by the point of articulation 31 and the center 66 of the articulation 65. This torque acts on the equilibrium of the steering system. In order to obtain good operation of the steering system, it is necessary suitably to control (for instance minimize) the combined effect of the torques Cp coming from the right and left struts.
Unfortunately, numerous vehicles still suffer today from poor suspension and/or steering characteristics. The dynamic behavior of a vehicle depends on a considerable number of parameters, some of which have an influence which is still poorly known but which may nevertheless be preponderant. The result is that even when the designer has duly respected all the rules of the art in designing the suspension, the result is not always up to the effort put into the design.